1. Field of the Invention
The present invention relates to an image forming apparatus that detects and controls the temperature of an object being heated by heating means. For example, it relates to an image forming apparatus that performs temperature control of a photosensitive member and a fixing unit.
2. Description of the Related Art
A typical image forming apparatus, such as a copier and a printer, uses a laser beam to form an electrostatic latent image on a photosensitive drum, which is an electrostatic latent image carrier. Such an apparatus makes the surface of the photosensitive drum electrified using an electrostatic charger and then sequentially performs exposure, development, transfer and fixing, thereby forming an image on a sheet of paper.
It is known that, in such an image forming apparatus, an ozone product is deposited on the surface of the photosensitive drum, and particularly in a high-humidity environment, an image deletion (a phenomenon that an image is blurred) occurs. In the case of a photosensitive drum that is relatively susceptible to wear, such as made of an organic photosensitive compound (OPC), the ozone product or the like can be relatively easily removed by polishing using abrasive means or the like. However, if the effect of the polishing is excessively high, the functionality of the photosensitive drum is deteriorated, and the service life thereof is shortened. On the other hand, there exists a photosensitive drum having the ozone product or the like deposited thereon which is hard to remove because of its high hardness, such as an amorphous silicon photosensitive drum.
Thus, in an electrophotographic image forming apparatus, a photosensitive drum heater is disposed in or near the photosensitive drum to control the surface temperature of the photosensitive drum approximately within a range of 35 to 45 degrees Celsius. While the temperature control of the photosensitive drum is carried out for various purposes, a primary purpose is to prevent or remove an image deletion occurring in a high-humidity environment. Ozone generated in a corona charger chemically modifies the surface of the photosensitive drum, and therefore, a hydrophilic group or the like is formed on the drum surface to provide the surface of the photosensitive drum with hygroscopicity. This causes an electrophotographic problem of a lateral shift of the surface potential of the photosensitive drum. Thus, the temperature of the photosensitive drum is controlled to remove the moisture on the surface that causes the problem. In addition, a material generated from ozone, such as NOx, is deposited on the surface of the photosensitive drum to provide the drum surface with hygroscopicity. Thus, the temperature control is carried out also to remove the moisture. In this way, an image deletion occurring in a high-humidity environment is prevented.
FIG. 13 is a diagram for illustrating temperature control of a photosensitive drum according to prior-art example 1.
In the example shown in FIG. 13, a photosensitive drum 2004 incorporates a heater 2001 that serves as a heating source, a thermistor 2002 that serves as temperature detection means, and a heater driver 2003 that controls the heater 2001 based on the output of the thermistor 2002. Reference numeral 2005 designates an AC power supply. Temperature control of the photosensitive drum 2004 is performed by detecting the temperature of the heater 2001 by the thermistor 2002 disposed in the heater 2001, rather than by detecting the surface temperature of the photosensitive drum.
FIG. 14 is a diagram for illustrating temperature control of a photosensitive drum according to prior-art example 2.
In the example shown in FIG. 14, a heater 3001 is disposed in a photosensitive drum 3002, and a non-contact thermistor 3003 is disposed close to the surface of the photosensitive drum 3002. Reference numeral 3005 designates an AC power supply. Temperature control of the photosensitive drum 3002 is performed by detecting the convection heat on the surface of the photosensitive drum 3002 by means of the thermistor 3003 and by controlling the heater 3001 by means of a heater driver 3004 disposed outside the photosensitive drum 3002.
This temperature control method has an advantage that the heating source can be controlled while detecting the surface temperature of the photosensitive drum to thereby detect any variation of the surface temperature of the photosensitive drum occurring in the course of operation of the image forming apparatus. However, in this temperature control method, a thermistor as temperature detection means is disposed out of contact with the surface of the photosensitive drum and detects the convection heat, rather than disposed in contact with the surface of the photosensitive drum, in order to prevent the thermistor from scratching the surface of the photosensitive drum. Consequently, the temperature detection accuracy is hard to improve because in addition to the fact that the performance of the thermistor itself is hard to improve, the detected temperature value is affected by the distance between the thermistor and the surface of the photosensitive drum. In addition, there is a problem of large temperature ripple because the response of the temperature detection is slow due to the heat capacity of the thermistor itself.
Thus, there has recently been contemplated that infrared temperature detection means is used which detects the amount of infrared radiation emitted from the surface of a temperature detection object to detect the temperature of the detection object. A representative one of such infrared sensors is a thermopile temperature sensor shown in FIG. 15.
FIG. 15 is a diagram showing an arrangement of a thermopile temperature sensor.
Referring to FIG. 15, a thermopile temperature sensor 4001 has a thermopile element 4002 comprising multiple thermocouples made of two different kinds of metals or semiconductor materials and connected in series to each other. A cold junction of the thermopile element 4002 is disposed in a heat sink 4003 that has a high heat capacity and provides for a reference, and a hot junction of the thermopile element 4002 is fixed to a member having a low heat capacity, and the thermopile element 4002 is covered with an infrared absorbing member 4004.
The infrared radiation emitted from the surface of the temperature detection object is collected through a lens 4005 of the thermopile temperature sensor 4001 and absorbed in the infrared absorbing member 4004. Alternatively, the infrared radiation from the object surface passes through a filter (not shown) disposed instead of the lens, and only part of the infrared radiation of a particular wavelength is absorbed in the infrared absorbing member 4004. Then, the thermopile element 4002 outputs a signal Sa corresponding to the temperature difference between the cold junction and the hot junction. Besides, a thermistor 4006 disposed at the cold junction detects the absolute temperature of the cold junction and outputs a signal Sb indicating the detected temperature. The signals Sa and Sb are input to a calculation circuit 4007, which produces a signal Sc that indicates the absolute surface temperature of the temperature detection object.
During manufacture of the thermopile temperature sensor 4001, the thermopile element 4002 and the thermistor 4006 are combined with the calculation circuit 4007, and they are adjusted so that the required detection accuracy can be achieved in a temperature zone where the sensor is actually used for the temperature detection. Thus, compared with the thermistor sensors shown in FIGS. 13 and 14, the temperature detection accuracy can be improved because the absolute surface temperature of the temperature detection object is detected.
Since such a thermopile temperature sensor has a high temperature detection accuracy, if the thermopile temperature sensor is used to detect the surface temperature of the photosensitive drum, there is a large latitude for an image deletion caused by a drop of the surface temperature of the photosensitive drum or for a failure due to melting/hardening of toner caused by a rise of the surface temperature of the photosensitive drum. Thus, the surface temperature of the photosensitive drum can be made more stable, and the image stability can be improved. In addition, the thermopile temperature sensor 4001 has an advantage that the response is quick compared with the non-contact thermistor, since the thermopile temperature sensor 4001 has a microstructure that enables rapid temperature detection.
Furthermore, in Japanese Laid-Open Patent Publication (Kokai) No. 2003-028721, there is proposed a method of improving the temperature detection accuracy in which a thermopile temperature sensor is used for a fixing unit. Furthermore, in Japanese Laid-Open Patent Publication (Kokai) No. 2000-259033, an image forming apparatus is proposed, in which a deviation error in temperature detection by a thermopile temperature sensor is corrected. That is, a contact thermistor with a contact/separation mechanism is provided for correction of the deviation error in temperature detection. The contact/separation mechanism keeps the thermistor separated from the fixing unit during normal operation and brings the thermistor into contact with the fixing unit when determining the deviation error in temperature detection, such as at the time of power-on.
However, even when the thermopile temperature sensor having a high temperature detection accuracy is used, if the required maintenance including lens cleaning is not adequately performed, the lens is soiled with paper dust or toner, the amount of infrared radiation passing through the lens decreases, and the detected temperature is shifted to lower temperatures. For example, when the thermopile temperature sensor is used for detecting the surface temperature of the photosensitive drum, if the temperature control is continued after the detected temperature is shifted to lower temperatures, the actual temperature of the photosensitive drum is higher than the detected temperature. As a result, toner can be molten and hardened on the developing sleeve that is in contact with the photosensitive drum, or the image stability can be deteriorated because the surface potential of the photosensitive drum varies due to the variation of the surface temperature of the photosensitive drum.
The technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-259033, which is designed to improve the accuracy of reference temperature (absolute temperature) detection by using a plurality of thermistors in addition to the thermopile element, is not effective against the problem of the shift of the detected temperature due to contamination of the lens. Thus, it is necessary for example to correct the temperature measured by the non-contact temperature sensor using the correcting temperature sensor.
The technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-259033 requires the contact/separation mechanism for the contact thermistor and, thus, has a problem concerning the installation space and cost of the contact/separation mechanism.
Furthermore, there has been developed an image forming apparatus that uses a thermopile temperature sensor for controlling the temperature of a fixing unit and has a cleaning member for cleaning the lens of the thermopile temperature sensor and an actuator that drives the cleaning member. This technique solves the problem of contamination of the lens of the thermopile temperature sensor but has a problem concerning the installation space and cost of the cleaning member, the actuator and the like.